The basic structure and the function of such a sheathed-element glow plug is recited in DE-PS 28 02 625 and U.S. Pat. No. 4,556,781. The sheathed-element of this sheathed-element glow plug contains a resistor element embedded in an insulating material, which is composed of two series-connected resistor spirals. The resistor spiral on the combustion chamber side of this resistor element is used as a heating element and has a resistance which essentially is independent of temperature, while the resistor spiral on the side removed from the combustion chamber has a high positive temperature resistance coefficient and acts as a regulating element; in this case the latter is made of nickel.
A glow plug is known from DE-PS 38 25 012 and U.S. Pat. No. 5,093,555 HAUSCH and Schieck assigned BERU RUPRECHT et al., which in principle has the same structure and the same function as the sheathed-element glow plug of the above mentioned DE-PS 28 02 625, but uses a cobalt-iron alloy or a nickel-iron alloy for the regulating element, where the content of iron is 20 to 25 weight percent. These two above mentioned alloys have a cubic body centered structure at room temperature, while they change into a cubic face centered structure when heated to a range between room temperature and 1000.degree. C. It has been shown that these last mentioned sheathed-element glow plugs only have a relatively short service life because of the breakdown of the material of the regulating element, lead to unwanted interruptions in operation and additionally entail expenses for correcting the trouble. The invention
In contrast thereto, the sheathed-element glow of the invention with the a cobalt-iron alloy resistor coil, whose iron content falls in a range between 6 and 18 weight-percent, present invention has the advantage that there is no breakdown of the regulating element of the resistor element after a relatively short length of operation and that service interruptions and expenses as a result thereof are avoided. This alloy maintains a cubic face-centered structure throughout a temperature range from not more than 0.degree. C. to at least 1000.degree. C.
Advantageous further developments of the sheathed-element glow plug are described below; it is particularly advantageous if the regulating element of the resistor element consists of a cobalt-iron alloy, the iron content of which is 12 to 14 weight percent.
Preferably, the regulating element is located in a section of the sheathed-element not radially surrounded by the metal housing.